This invention relates to an optical glass and, more particularly, to an optical glass which is stable and free of poisonous ingredients and has a refractive index (Nd) within a range of 1.71 - 1.80 and Abbe number (.nu.d) greater than values on a line connecting a point where Nd is 1.71 and .nu.d is 55.0 and a point where Nd is 1.80 and .nu.d is 48.0 in the co-ordinates shown in the accompanying drawing in which the ordinate represents the refractive index (Nd) and the abscissa represents Abbe number (.nu.d).
The optical glasses having the above described optical properties, that is, having a high refractivity and a great Abbe number (i.e. low dispersion), are very useful for the designing of optical instruments so that various developments have been made to obtain such optical glasses. Known in the art of such optical glasses are those of a B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -ThO.sub.2 system as disclosed in the specification of U.S. Pat. No. 2,434,146 and those of a B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -Ta.sub.2 O.sub.5 -ThO.sub.2 as disclosed in the specification of U.S. Pat. No. 2,150,694. These known optical glasses, however, are not suitable for commercial production since they contain ThO.sub.2 which is poisonous to humans because of its radioactivity and also are not sufficiently stable and has a tendency to devitrification. For eliminating these disadvantages, glasses of a B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -Gd.sub.2 O.sub.3 -Ta.sub.2 O.sub.5 system as disclosed in the specification or British patent No. 1,183,996, those of a B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -Gd.sub.2 O.sub.3 -RO (wherein RO is a divalent metal oxide) system and those of a B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -Gd.sub.2 O.sub.3 -Ta.sub.2 O.sub.5 -RO system as disclosed in the specification of U.S. Pat. No. 3,958,999 have been developed. These optical glasses, however, are disadvantageous in that the above described desired optical properties can hardly be obtained.